Device and method for measuring the viscoelastic properties of a viscoelastic medium

ABSTRACT

A device for vibration controlled transient elastography, in particular to quantify liver fibrosis, includes an ultrasound probe for elastography comprising a probe casing, at least one ultrasound transducer having a symmetry axis, a vibrator, and a force sensor, wherein the vibrator is arranged to induce a movement of the probe casing along the symmetry axis of the ultrasound transducer, the ultrasound transducer being bound to the probe casing with no motion of the ultrasound transducer relative to the probe casing, and wherein the device includes a signal generator configured to issue a contact ready signal when the force applied by the probe on the to-be-measured viscoelastic medium is greater than a minimum contact force threshold. The signal generator may further be configured to issue a measurement ready signal when the force is greater than a minimum measurement force threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/498,162, filed Sep. 26, 2019, now U.S. Pat. No. 11,331,073, which isthe U.S. National Stage of PCT/EP2018/057609, filed Mar. 26, 2018, whichin turn claims priority to European Patent Application No. 17163075.9filed Mar. 27, 2017, the entire contents of all applications areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a device for measuring the viscoelasticproperties of a viscoelastic medium such as a human or an animal organ.More specifically, the present invention can be used to measure theviscoelastic properties of a liver which permits to quantify the amountof fibrosis present in said liver. The invention also concerns a methodfor measuring the viscoelastic properties of a viscoelastic medium.

BACKGROUND

Chronic hepatitis, which can be of alcoholic, viral or other origin, hasa fibrotic effect that it is important to evaluate so that to determinethe best time to treat the hepatitis.

One of the most reliable and efficient techniques to measure liverstiffness is transient elastography (see for example “WFUMB guidelinesand recommendations for clinical use of ultrasound elastography part 3:liver” by G. Ferraioli et al. published in “Ultrasound in Med. AndBiol.”, 41, 5, 2015).

The applicant has developed and commercialized a device calledFibroscan® (see for example patents EP1169636 and EP1531733). Thisdevice measures the stiffness of the liver by using an elastographytechnique called “Vibration Controlled Transient Elastography” (VCTE)developed by the applicant.

In a VCTE application, the measure of the liver stiffness relies on themeasure of a transient shear wave propagation speed inside the tissue.

In order to perform such a measurement, a particular probe has beendeveloped. Said probe comprises at least an electrodynamical actuatorand at least an ultrasound transducer mounted on the tip of the probe.

For example, in the Fibroscan® probe, the vibrator moves the ultrasoundtransducer and pushes it against the patient body. This pulsed movementgenerates a transient shear wave which propagates inside the liver. Thedisplacement generated by the propagating shear wave is probed bysending high frequency ultrasound short pulses or shots inside themedium.

Thanks to the advantageous geometry used by the Fibroscan® (see forexample FIG. 1 a ) the mechanical actuator and the ultrasound transducershare the same symmetry axis, as indicated by the dashed line of FIG. 1a . This geometrical arrangement makes it possible to avoid systematicerrors in the measurement of the shear wave propagation speed: the shearwave and the ultrasound shots propagate along the same direction.

Moreover, the Fibroscan® probe comes with a motion sensor capable ofmeasuring the displacement of the probe tip with respect to the probecasing, for example a Hall effect position sensor. A measurement isvalidated only if the tip trajectory follows a predetermined profile,for example a period of sinusoid. In traditional VCTE probe only therelative movement of the probe tip with respect to the probe casing ismeasured. In other words, in a traditional VCTE probe the movement ofthe probe tip is measured in the reference frame of the probe casing.

A relevant problem in currently available VCTE probes is the control ofthe real movement of the probe tip when applying the transient shearwave to the tissue to be examined. For example, when the shear wave isapplied, the recoil of the probe can add to the movement of the tip andthe applied pulse can be deformed. This problem is related to the recoilof the operator's hand and to the control of the force which theoperator has to apply in holding the probe against the patient's body:for a correct use of a VCTE probe according to the prior art a skilledor qualified operator is needed.

If the probe recoil is not controlled, the real movement of the tip ofthe probe with respect to the patient's body is unknown. The measurementcan be dependent upon the force applied by the operator during thegeneration of the shear wave.

An ultrasound probe comprising a linear actuator and a force sensor isdisclosed by patent applications U.S. Pat. No. 8,333,704 B2 (“Hand-heldforce-controlled ultrasound probe” filed by Anthony et al. on 18 Dec.2010) and US 2012/0316407 A1 (“Sonographer fatigue monitoring” filed byAnthony et al. on Dec. 6, 2012). FIG. 1 b illustrates the devicedescribed by the document U.S. Pat. No. 8,333,704. According to thesedocuments, the ultrasound transducer is moved by an electrodynamicalactuator in order to control the force applied against the tissue to beanalyzed. The movement of the ultrasound transducer is controlled basedon the signal provided by the force sensor and used as a feedbacksignal.

These documents solve the problem of applying a constant or atime-dependent force during an ultrasound measurement but the disclosedtechnical solutions have several drawbacks.

For example these devices described in the prior art comprise anexternal mechanical mobile part: as it is shown by the arrow in FIG. 1 bthe ultrasound transducer is moved with respect to the probe casing.Several drawbacks are associated to this external moving part, forexample the need for frequent calibration operation.

Another solution described in the prior art is described in the document“Probe Oscillation Shear Elastography (PROSE): A high Frame-Rate Methodfor Two-dimensional ultrasound shear wave elastography” by D. Mellema etal. (published in IEEE Transaction on medical imaging, Vol. 35, No 9,September 2016). In this case a probe for continuous wave elastographyis described. This solution is not adapted for the application of atransient shear pulse to the tissue but only for continuous wave shearwave oscillation. Moreover the described elastography probe is formed bytwo separate components, which is a severe drawback for the in-vivoapplications. For example the described probe is difficult to manipulatedue to the presence of several parts.

Moreover, according to operators, when the ultrasound transducer isplaced on the patient's body, it is difficult to control the appliedforce. Thus, such a device does not permit accurate and reproduciblemeasurements regardless of the operator.

Moreover, if the applied force on the patient's body is too high, theelectrodynamic actuator of the device may be damaged.

Finally, the acoustic power delivered to patients by the ultrasoundtransducer of the sensor can be high and unnecessarily used which canlead to patient injury, electronic damage and ultrasound transducerpremature failure.

SUMMARY

An aspect of the invention is directed to a device that overcomes theaforementioned drawbacks. Accordingly, an aspect of the invention isdirected to a device for accurate and reproducible measurements of theviscoelastic properties of a viscoelastic medium which limits health'srisks for patients and prolongs the lifetime of the device.

To achieve this, a first aspect of the present invention is directed toa device for measuring viscoelastic properties of a viscoelastic mediumhaving an ultrasound signal after being subjected to ultrasound pulsescomprising:

-   -   a probe for transient elastography comprising:        -   a probe casing;        -   at least one ultrasound transducer having a symmetry axis;        -   at least a vibrator, said vibrator being located inside the            probe casing;        -   a force sensor, the force sensor being configured to measure            a force applied by the probe against the to-be-measured            viscoelastic medium;    -   a signal generator;    -   the device being characterized in that:        -   the vibrator is arranged to induce a movement of the probe            casing along a predefined axis, the predefined axis being            the symmetry axis of the ultrasound transducer;        -   the ultrasound transducer is bound to the probe casing with            no motion of the ultrasound transducer with respect to the            probe casing;        -   the signal generator is constructed and arranged to issue a            contact ready signal when the ultrasound transducer of the            probe is in contact with a to-be-measured viscoelastic            medium, the contact ready signal being set by the signal            generator when the force applied by the ultrasound probe on            the viscoelastic medium is greater than a minimum contact            force threshold.

A probe casing is the enclosure of the VCTE probe, said enclosurecontaining a vibrator or electrodynamical actuator. The probe casing cancontain also other elements as a position sensor, logic circuits orconnecting means in order to store data or exchanging data with acomputer or other electronic devices. An ultrasound transducer is adevice adapted to emit and receive ultrasound waves. It is formed by asingle transducer or by an array of transducers, forming for example alinear detector.

A symmetry axis of the ultrasound transducer is an axis of geometricalsymmetry of the transducer. The symmetry axis of the ultrasoundtransducer is also the direction along which the ultrasounds are emittedby the transducer. The symmetry axis of the transducer corresponds tothe propagation direction of the ultrasound short pulses emitted by thetransducer.

According to the present invention, the ultrasound transducer is boundin motion to the probe casing, which means that there is no relativemovement of the ultrasound transducer with respect to the probe casing.

An extremity of the ultrasound transducer can be fixed to an extremityof the probe casing. Another extremity of the ultrasound transducer isfree to vibrate in order to transmit the ultrasound waves to the mediumto analyze.

Alternatively, the ultrasound transducer can be attached to the probecasing by means of a probe tip. When present, the probe tip has anextremity fixed to one extremity of the probe casing and anotherextremity fixed to the ultrasound transducer.

The explication of the invention given in the following paragraphs holdsboth when the ultrasound transducer is fixed directly to the probecasing and when the ultrasound transducer is fixed to the probe casingthrough a probe tip. These two configurations are given only asexemplary embodiments and other configurations are possible.

A vibrator is a device adapted to move a mass inside a probe casing. Forexample the vibrator can oscillate a mass at a frequency comprisedbetween 1 and 5000 Hz.

The probe according to the invention can be considered as an inertialprobe because the movement of the probe itself is generated by themovement of a mass inside the probe casing. The probe according to theinvention comprises no external mechanical moving part.

The probe according to the invention is a transient elastography probe.This means that it is adapted to both applying a transient shear wave tothe tissue to detect and analyze the propagation of the shear wave bysending ultrasound short pulses at high repetition rate.

The shear wave is generated in the tissue by the movement of the probe,which pushes the ultrasound transducer against the tissue itself. Theshear wave is generated by applying a low frequency pulse to the surfaceof the medium. For example, the pulse can have the shape of one periodof a sinusoid at a central frequency f comprised between 1 Hz and 5000Hz.

The duration of the low frequency pulse applied to the tissue iscomprised between ½f and 20/f.

Ultrasound short pulses are emitted at a repetition rate comprisedbetween 100 Hz and 100*10³ Hz.

The propagation of the shear wave is detected by sending ultrasoundpulses or shots at high repetition rate inside the medium and bydetecting the backscattered ultrasound signals. In fact the tissuecontains inhomogeneity or particles capable of partially reflecting theultrasound pulses.

By recording and analyzing subsequent backscattered signals it ispossible to compute the displacement of the tissue due to thepropagation of the shear wave. The properties of the shear wave can thenbe deduced. For example, the propagation speed of the shear wave isdirectly related to the stiffness of the viscoelastic medium.

An advantage of the probe described in the present invention is tocontrol the real movement of ultrasound transducer. In fact, theultrasound transducer is bound to the probe casing and there is noreciprocal movement between the ultrasound transducer and the probecasing. It is then possible to monitor the movement of the probe casing,which corresponds to the real movement of the ultrasound transducer.Measuring the real movement of the ultrasound transducer is important inorder to control the shape of the transient shear wave generated insidethe tissue. For example the movement of the probe can be measured withan accelerometer mounted on the probe itself.

In other words, according to the present invention, the movement of theultrasound transducer is measured in the reference frame of the earth,in contrast to what is done in a traditional VCTE probe. In fact in aVCTE probe according to the prior art, the movement of the ultrasoundtransducer is measured in the reference frame of the probe casing andonly the relative movement of the ultrasound transducer with respect tothe probe casing is measured.

In practice, the motion of the mass actuated by the vibrator inside theprobe casing can be determined by a control loop using the movement ofthe probe casing as a feedback signal. This makes it possible todirectly compensate the motion of the hand of the operator during theapplication of the shear wave.

There is no need for the operator to apply a precise force in order tocompensate the recoil of the probe. As a consequence, performing ameasure of stiffness on a viscoelastic medium becomes easier for theoperator of the probe. Moreover, the measured values of stiffness aremore reproducible.

By measuring only the relative movement of ultrasound transducer withrespect to the probe casing, as it is done in the prior art, it wouldnot be possible to take into account the recoil of the probe. As aconsequence, even if the relative movement of the tip follows asinusoidal trajectory, the effective low frequency pulse applied to thepatient body can have a different shape due to the recoil of the probe.

According to the present invention the ultrasound transducer in contactwith the patient's body moves together with the probe casing. Detectingthe probe casing movement is equivalent to detecting the probe tipmovement. The probe casing movement is used as a feedback for thevibrator. In fact the amplitude of the oscillation of the vibrator canbe adjusted in order to obtain the desired movement of the probe tip incontact with the patient's body. Moreover, the lack of external movingpart in the probe according to the invention removes the need forfrequent mechanical calibration.

As the device of the invention comprises a signal generator that issuesa contact ready signal, the operator does not activate the ultrasoundsignals unless the signal generator issues a contact ready signal. Thus,said device guarantees that the ultrasound signal power emitted by theultrasound transducer is used only when necessary and is as low aspossible for the patient.

Said contact ready signal is issued by the signal generator based on thecontact force between the ultrasound transducer and the patient's body.The contact force can be measured by a force sensor placed on the probe.The contact ready signal is issued only if the measured contact forcemeets a predefined condition. For example, the measured contact forcemust be greater than a minimum contact force threshold. When such acondition is met the probe is considered in contact with theto-be-measured viscoelastic medium. Alternatively, the contact readysignal can be issued only if the contact force is comprised between aminimum contact force threshold and a maximum contact force threshold.

Moreover, considering that the contact ready signal is set only when thetransducer of the ultrasound probe is in contact with the to-be-measuredviscoelastic medium, the device's measurements are accurate andreproducible regardless of the device's operator. In fact, with thedevice of the prior art, the operator can generate ultrasound signalseven when the transducer attached to the tip of the ultrasound probe isnot in contact with the to-be-measured medium, which causes inaccurateor false measurements. Indeed, the measurements realized with the deviceof the prior art depend on the operator and more particularly on thecontact of the probe against the patient's body.

In addition to the contact ready signal, the device according to theinvention is constructed to emit a measurement ready signal. Themeasurement ready signal is emitted only when the contact force betweenthe US transducer and the tissue is comprised between a minimum and amaximum measurement force threshold. When this condition is verified,the measurement ready signal is issued and a viscoelastic measurement istriggered, automatically or manually. In other words, a low frequencypulse is applied to the tissue only if the measurement ready signal isissued.

The device according to the first aspect of the invention may also haveone or more of the features below, considered individually or accordingto all of the technically possible combination:

-   -   The signal generator is constructed and arranged to issue a        measurement ready signal;    -   The force sensor is constructed and arranged to measure a force        applied by the ultrasound probe against the to-be-measured        viscoelastic medium, the contact ready signal being set by the        signal generator when the force applied by the ultrasound probe        on the viscoelastic medium is greater than a minimum contact        force threshold and the measurement ready signal being set by        the signal generator when the force applied by the ultrasound        probe is greater than a minimum measurement force threshold;    -   the minimum contact force threshold is comprised between 0.1 N        and 1.0 N;    -   the minimum measurement force threshold is comprised between 1.0        N and 6.0 N;    -   the measurement ready signal is set by the signal generator when        the force applied by the ultrasound probe on the viscoelastic        medium is smaller than a maximum measurement force threshold;        -   the maximum measurement force threshold is comprised between            6.0 N and 20.0 N;    -   The minimum contact force threshold is equal to 0.5 N;    -   The minimum measurement force threshold is equal to 4.0 N;    -   The maximum measurement force threshold is equal to 8.0 N;    -   The contact ready signal is set when the force measured by the        force sensor is comprised between a minimum contact force        threshold and a maximum contact force threshold;    -   The device according to one of the previous claims characterized        in that the ultrasound transducer is bound to the probe casing        by means of a probe tip, said probe tip having a first extremity        fixed to the probe casing and a second extremity fixed to the        ultrasound transducer;    -   The device according to the previous claim characterized in that        the probe tip is interchangeable;    -   The probe comprises a position sensor and the device comprises a        control loop configured to control the vibrator based on the        signal received from the position sensor;    -   the force sensor is a capacitive sensor or an applied force        sensor;    -   the device comprises means to trigger a measurement of a        viscoelastic property of a viscoelastic medium only if a        measurement ready signal is set; these means comprise an        electronic microchip or an electronic microprocessor receiving        the contact ready signal and the measurement ready signal; if an        acquisition is required, the microchip or microprocessor        triggers the measure of a viscoelastic property when the        measurement ready signal is set;    -   the vibrator or electrodynamic actuator is constructed and        arranged to generate low-frequency impulse displacements of the        ultrasound probe only when the measurement ready signal is set;    -   the force sensor comprised in the probe is constructed and        arranged to measure a force applied by the ultrasound probe        against the to-be-measured viscoelastic medium, the contact        ready signal and/or the measurement ready signal being set by        the signal generator when the force measured by the force sensor        is:        -   for the contact ready signal, superior to a minimum force            contact threshold,        -   for the measurement ready signal, superior to a minimum            force measurement threshold. Further, in a not limited            embodiment, the measurement ready signal is set when the            force measured by the force detecting module, is inferior to            a maximum force measurement threshold;    -   The ultrasound transducer is constructed and arranged to        activate the emission of the ultrasound signals when the contact        ready signal is set;    -   The device comprises a display unit which displays ultrasound        images, the display unit being constructed and arranged to        refresh ultrasound images only when the contact ready signal is        set;    -   The device comprises guidance indicators constructed and        arranged to refresh only when the contact ready signal is set;    -   The ultrasound probe comprises at least one light-emitting        diode, the ultrasound probe being constructed and arranged to        light the light-emitting diode only when the contact ready        signal is set;    -   The device comprises device commands, the access to the device        commands being possible only when the contact ready signal is        set;    -   The device is constructed and arranged to deactivate a combined        modality of said device when the contact ready signal is set;    -   The device is constructed and arranged to activate a combined        modality of said device when the contact ready signal is set;    -   The ultrasound transducer is bound to the probe casing by means        of a probe tip, said probe tip having a first extremity fixed to        the probe casing and a second extremity fixed to the ultrasound        transducer;    -   The device is constructed and arranged to trigger the        application of a low frequency pulse to the to-be-measured        viscoelastic medium only if the measurement ready signal is set;    -   The probe comprises a position sensor and the device comprises a        control loop configured to control the vibrator based on the        signal received from the position sensor;    -   The force sensor is a capacitive sensor or an applied force        sensor;    -   A second aspect of the present invention is directed to a method        for measuring viscoelastic properties of a viscoelastic medium        having an ultrasound signal after being subjected to ultrasound        impulses comprising the following steps:        -   positioning of an ultrasound transducer of an ultrasound            probe in contact with a to-be-measured viscoelastic medium,        -   generating a contact ready signal when the transducer of the            ultrasound probe is in contact with the to-be-measured            viscoelastic medium, the contact ready signal being issued            by a signal generator constructed and arranged to issue a            contact ready signal.

The method according to one aspect of the invention may also have one ormore of the features below, considered individually or according to allof the technically possible combination:

-   -   The method comprises a further step of measuring a force applied        by the ultrasound probe against the to-be-measured viscoelastic        medium, the measurement of a force being determined by a force        sensor constructed and arranged to measure a force applied by        the ultrasound probe against a to-be-measured viscoelastic        medium, the contact ready signal being set by the signal        generator when the force exerted by the ultrasound probe is        higher than a minimum contact force threshold and the        measurement ready signal being set by the signal generator when        the force applied by the ultrasound probe is higher than a        minimum measurement force threshold or comprised between a        minimum and a maximum measurement force threshold.    -   The method comprises a step of emitting ultrasound pulses only        when the contact ready signal is issued;    -   The method comprises a step of displaying localization means        only if the contact ready signal is issued; localization means        are tools used by an operator in order to locate the        to-be-measured viscoelastic tissue; example of localization        means are imaging, guidance tools or other indicators;    -   The method comprises a step of refreshing images only if the        contact ready signal is issued;    -   The method comprises a step of refreshing the guidance tools        only if the contact ready signal is issued;    -   The method comprises a step of accessing the memory of the        device only if the contact ready signal is issued;    -   The method comprises a step of setting on a led only when the        contact ready signal and the measurement ready signal are set;    -   The method comprises a step of triggering a measurement of a        viscoelastic properties only when the contact ready signal and        the measurement ready signal are set;    -   The method comprises a step of accessing a command only when the        contact ready signal and the measurement ready signal are set;    -   The method comprises a step of deactivate other modality when        the contact ready signal is issued and a step of activate other        modality when the contact ready signal is not issued;

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in a constitute partof this specification, to illustrate aspects of the invention and,together with the description, to explain the principles of theinvention:

FIG. 1 a represents a transient elastography device according to theprior art;

FIG. 1 b represents a force controlled ultrasound probe according to theprior art;

FIG. 2 represents an example of a device for measuring the viscoelasticproperties of a viscoelastic medium according to an aspect of theinvention.

FIG. 3 represents an embodiment of the device according to FIG. 2 ;

FIG. 4 represents the steps of a method for measuring the viscoelasticproperties of a viscoelastic medium according to an aspect of theinvention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

This invention provides, in a first aspect, a device DEV for measuringthe viscoelastic properties of a viscoelastic medium having anultrasound signal after ultrasound illumination. More particularly, thedevice DEV of the invention permits to quantify instantaneously and in anon-invasive manner a liver fibrosis.

According to an example selected to illustrate an aspect of theinvention and illustrated in FIG. 2 , the device DEV comprises anultrasound probe 1. FIG. 2 shows a sectional view of the probe 1.

The ultrasound probe 1 comprises:

-   -   a probe casing PC containing at least a vibrator or an        electrodynamical actuator VIB; in a particular embodiment the        vibrator VIB comprises a fixed part FIX and a mobile part MOV;    -   a first vertical bar F1, a second vertical bar F2 and a        horizontal bar F configured to fix the fixed part FIX of the        vibrator VIB to the probe casing PC;    -   an ultrasound transducer US having a symmetry axis A;    -   a position sensor POS comprising an accelerometer ACC and        configured to measure the position or the displacement of the        probe casing PC as a function of time, said position sensor        cooperating with a control loop analyzing the data provided by        the position sensor POS and controlling the vibrator VIB; the        position sensor is used to control the vibration of the probe as        a function of acceleration, speed or preferably, displacement or        position;    -   a probe tip PT having a first extremity PTE1 fixed to the front        end of the probe casing PC and a second extremity PTE2 fixed to        the ultrasound transducer US, the front end of the probe casing        PC being the extremity of the probe casing which is placed at        proximity of the tissue;    -   a force sensor FS placed on the probe tip PT and in proximity of        the ultrasound transducer US, in between the probe tip and the        rest of the probe, said force sensor FS being connected to a        signal generator 9 by connecting means not showed in FIG. 1 ;        the force sensor FS is constructed and arranged to measure the        contact force exerted by the probe 1 on the viscoelastic tissue        to be measured;    -   connecting means for connecting the position sensor POS to the        control loop and to the vibrator VIB, connecting means for        connecting the ultrasound transducer US and the force sensor FS        with the other components of the device DEV;    -   a display unit 7 constructed and arranged to display ultrasound        images,    -   a push-button 8 constructed and arranged to turn on the        ultrasound probe 1,    -   a signal generator 9 constructed and arranged to issue a contact        ready signal or a measurement ready signal or both a contact        ready signal and a measurement ready signal.    -   The device DEV further comprises:        -   device commands 10 constructed and arranged to control the            device DEV by an operator,        -   a cable 11 constructed and arranged to link the ultrasound            probe 1 to the device commands 10.

In the following description, we will take the example of a liver as theviscoelastic medium whose viscoelastic properties are measured.

As specified, the signal generator 9 of the device DEV of the inventionis constructed and arranged to emit a contact ready signal when thetransducer US of the ultrasound probe 1 is in contact with the to-bemeasured viscoelastic medium. The generation of the contact ready signalmakes it possible to obtain accurate and reproducible measurements ofthe viscoelastic properties of a liver regardless of the device'soperator.

According to an aspect of the invention, the contact ready signal isdetermined by using the force sensor FS. Actually, a contact readysignal is generated by the signal generator 9 when the force applied bythe ultrasound probe 1 against the patient's skin is greater than aminimum contact force threshold. More particularly, the force sensor FSis adapted to measure the force exerted by the probe 1 against thepatient's skin.

According to an embodiment, the contact ready signal is issued by thesignal generator 9 when the force measured by the force sensor FS isgreater than a minimum contact force threshold.

According to an embodiment the minimum contact force threshold iscomprised between 0.1 N and 1 N.

According to an embodiment the minimum threshold for the contact forceis 0.5 N. This minimum force level is used to detect a contact betweenthe tip of the probe and the to-be-measured viscoelastic tissue.

Advantageously, the use of a minimum contact force threshold makes itpossible to control the contact between the probe and the viscoelasticmedium in order to avoid, for example, the emission of ultrasounds whenthe probe is not used.

According to an embodiment, the contact ready signal is issued by thesignal generator 9 when the force measured by the force sensor FS issmaller than a maximum contact force threshold.

Advantageously, the contact ready signal makes it possible to preventthe operator from applying an excessive force against the patient's bodyand therefore hurting the patient.

Advantageously, the contact ready signal makes it possible to preventthe damaging of the probe 1 due to the high force exerted against thepatient's body.

According to an embodiment, the measurement ready signal is issued bythe signal generator 9 when the force measured by the force sensor FS isgreater than a minimum measurement force threshold.

Advantageously, the minimum measurement force threshold is necessary inorder to efficiently transmit a shear wave into the viscoelastic mediumand to obtain a reliable measurement of the viscoelastic properties ofthe tissue.

According to an embodiment of the present invention the measurementready signal is issued by the signal generator 9 when the force measuredby the force sensor FS is comprised between a minimum measurement forcethreshold and a maximum measurement force threshold.

According to an embodiment the minimum measurement force threshold iscomprised between 1.0 and 6.0 N and the maximum measurement forcethreshold is comprised between 6.0 N and 20.0 N.

Advantageously this range of force thresholds makes it possible to adaptthe measurement conditions to the size of the ultrasound transducer andto the body-type of the patient. For example in the case of an obesepatient a larger ultrasound probe and higher force thresholds can bechosen in order to correctly apply a low frequency pulse for anelastography measurement.

According to an embodiment, the minimum measurement force threshold isequal to 4 N and the maximum measurement force threshold is equal to 8N.

A viscoelastic measurement is triggered only if the measurement readysignal is issued by the signal generator 9. Advantageously, when thecondition on the measurement force threshold is verified and themeasurement ready signal is issued, the low frequency pulse isefficiently applied to the viscoelastic tissue and its shape isprecisely controlled.

The device DEV comprises means to trigger a measurement of aviscoelastic property of a viscoelastic medium only if a measurementready signal is set; these means comprise an electronic microchip or anelectronic microprocessor receiving the contact ready signal and themeasurement ready signal; if an acquisition is required, the microchipor microprocessor triggers the measure of a viscoelastic property whenthe measurement ready signal is set. The means to trigger a measurementcan be embedded in the device commands 10.

Advantageously, when the condition on the measurement force threshold isverified and the measurement ready signal is issued, the shear wave isefficiently induced into the patient's body given that the impedance ismatched in between the subcutaneous tissues and the liver.

According to an embodiment of the present invention the force sensor FSis a capacitive sensor or an applied force sensor.

An advantage of this embodiment is to precisely measure the forceexerted from the probe 1 against the patient's body.

According to an embodiment of the present invention the probe 1comprises a position sensor POS and the device DEV comprises a controlloop configured to control the vibrator (VIB) based on the signalreceived from the position sensor (POS).

The control loop can be embedded in the probe 1 or in the devicecommands 10. In practice the control loop sets the motion parameters ofthe vibrator VIB in order to obtain a target low frequency pulse. Theposition of the probe 1 as measured by the position sensor POS is usedas a feedback signal for the control loop.

An advantage of this embodiment is to precisely control the shape of thelow frequency pulse applied to the patient's body. Moreover the probe 1has no external moving parts which eliminates the need for frequentmechanical calibrations.

FIG. 3 is a detailed representation of a sectional view of probe 1further comprising a first spring K1 extending from the first bar F1 tothe mobile part MOV and a second spring K2 extending from the second barF2 to the mobile part MOV;

According to an embodiment, the probe casing PC has a cylindrical shape,the axis A being the axis of the cylinder. Alternatively, the probecasing can have the shape of a solid of revolution having axis A.

The size of the probe casing is chosen in order to obtain a handheldprobe. According to the embodiment the circumference of the cylinder iscomprised between 120 mm and 160 mm.

The axis A is the symmetry axis of the ultrasound transducer US. Forexample in the case of a cylindrical ultrasound transducer the axis A isthe axis of the cylinder forming the transducer. The axis A identifiesalso the propagation direction of the ultrasound short pulses emitted bythe ultrasound transducer US.

According to another embodiment, the probe casing PC can have any shapeadapted to be held by the operator's hand during the measurement. Forexample the probe casing PC can have the shape of a standard echographyprobe as it is showed in FIG. 5 .

The vibrator VIB is placed inside the probe casing PC and it is formedby two elements: a mobile mass MOV and a fixed element FIX. The vibratorVIB is configured to set the mass MOV in motion, which generates themotion of the whole probe 1 along the axis A.

We define as vertical a direction normal to the axis A and as horizontala direction parallel to the axis A.

According to an embodiment, the fixed part FIX is held in place by theholding means formed by a first vertical bar F1, a horizontal supportbar F and a second vertical bar F2. The first and second vertical barsF1 and F2 are fixed to the probe casing. The horizontal support bar Fextends from the first vertical bar F1 to the second vertical bar F2.

Alternatively, only one vertical bar, F1 or F2, can be present in orderto support the horizontal bar F and the vibrator VIB.

The holding means F1, F and F2 block the fixed part FIX that is thenbound to the probe casing PC. Any other configuration of holding meansadapted to bind the fixed part FIX of the vibrator VIB to the probecasing PC can be used.

The mobile part MOV is separated from the first and second verticalbars, respectively F1 and F2, by two springs, respectively K1 and K2.The first spring K1 extends from the first vertical bar F1 to the movingpart MOV, the second spring extends from the second vertical bar F2 tothe moving part MOV.

When actuated by the vibrator VIB, the moving part MOV slides along thehorizontal bar F. The horizontal bar F supports both the fixed part FIXand the mobile part MOV of the vibrator VIB.

The two springs K1 and K2 support the moving part MOV and act as arecalling force when the moving part MOV is set in motion.

It is worth to note that the moving part MOV oscillates inside the probecasing PC. The vibrator VIB does not move any external part of theinertial probe 1.

According to the embodiment represented in FIG. 2 , the mobile mass MOVis a permanent magnet and the fixed part FIX is a coil. When an electricpotential is applied to the coil FIX, a force is exerted between thecoil FIX and the magnet MOV and an oscillation of the mass MOV along theaxis A is produced.

The movement of the probe casing PC is induced by the movement of themoving part MOV due to both the action of the electromagnetic forcebetween the coil and the magnet and the recalling force exerted by thesprings K1 and K2. This movement can be described as consequence of thelaw of conservation of momentum, the movement of the moving part MOVdetermining the recoil of the probe casing PC.

As a result, the whole inertial probe 1 is set in motion and theultrasound transducer US is pushed against the patient's body.

An advantage of this configuration is that the movement of theultrasound transducer US against the tissue to analyze is directlydetermined by the vibrator VIB and it can be accurately controlled. Inother words, given the absence of relative movement of the ultrasoundtransducer US with respect to the probe casing PC, the amplitude of thedisplacement of the ultrasound transducer US coincides with theamplitude of the movement of the probe casing PC. The shape of the lowfrequency pulse applied to the tissue is then accurately controlled.

According to the invention, there are several possible solutions to fixthe ultrasound transducer US to the probe casing PC.

According to an embodiment, the ultrasound transducer US can be directlyfixed to the probe casing PC. Alternatively the ultrasound transducer UScan be fixed to a force sensor FS which is in turn attached to the probecasing PC.

An advantage of this embodiment is that this configuration is simple torealize. Moreover the force sensor FS is directly in contact with theultrasound transducer US, which makes the detection of the probe casingPC deformation more efficient. The deformation of the probe casing PC isa micrometric deformation and it is due to the contact between theultrasound transducer US and the tissue to be analyzed.

According to the embodiment represented in FIG. 2 , the ultrasoundtransducer US is fixed to a probe tip PT which comprises a firstextremity PTE1. The first extremity PTE1 is fixed to the front end ofthe probe casing PC. For example the probe tip PT has a sensiblycylindrical shape as it is shown in FIG. 1 .

For example, the probe tip PT can be secured to the probe casing PC byinserting the first extremity of the probe tip PTE1 inside a housing HOUpresent in the force sensor FS, as it is showed in FIG. 2 . The secondextremity of the probe tip PTE2 is fixed to the ultrasound transducerUS.

An advantage of this embodiment is that the probe tip PT is easilyinterchangeable. In other words it is possible to use different probetips PT having different ultrasound transducer US, in order to adapt theproperties of the emitted ultrasound shots to the properties of thetissue or of the patient's body.

According to an embodiment, the motion of the inertial probe 1 ismeasured by means of a position sensor POS.

An advantage of this embodiment is the direct measurement of theamplitude of the movement of the probe casing PC, which is identical tothe amplitude of the movement of the ultrasound transducer US. In factaccording to the invention no movement of the ultrasound transducer USwith respect to the probe casing PC is possible. In other words, theultrasound transducer US is at rest in the reference frame of the probecasing PC.

In the embodiment represented in FIG. 3 the position sensor POS isformed by an accelerometer ACC and an electronic circuit DI performing adouble temporal integration. The double integrator DI gives the positionr of the probe starting from the measured acceleration.

Any electronic circuit capable of computing the position r from themeasured acceleration can be used in the present invention.

Advantageously, the position sensor POS provides a direct measurement ofthe displacement of the ultrasound transducer US. In other words theposition sensor POS directly measures the shape of the low-frequencypulse applied to the tissue in order to generate the transient shearwave inside the tissue.

The probe 1 is then adapted to cooperate with a control loop capable ofdriving the vibrator VIB in order to obtain a predefined low frequencypulse shape. The control loop can for example be embedded in aFibroscan® device.

The position r measured by the position sensor POS is then used as afeedback signal for controlling the vibrator VIB. According to anembodiment, the position r is fed to a control loop which controls theamplitude and frequency of the oscillation of the moving mass MOV.

Thanks to this arrangement, the movement of the ultrasound transducer UScan be directly controlled and a well-defined low frequency pulse isapplied to the patient's body.

According to the embodiment showed in FIG. 3 the probe 1 comprises alsoconnecting means for transmitting the electrical signals between theposition sensor POS, the control loop and the vibrator VIB. Theconnecting means comprise also means for delivering the electrical powernecessary in order to operate the vibrator VIB, the ultrasoundtransducer US and the other embedded equipment. The power deliveringmeans are not showed in FIG. 3 .

According to the embodiment showed in FIG. 3 , the force sensor FS is atip holder TH equipped with at least one strain gauge SG or anotherstress sensor. The force sensor of FIG. 3 is further adapted to receiveand secure an extremity of the probe tip PT.

According to an embodiment, the means connecting the position sensorPOS, the control loop and the vibrator VIB can be wireless.

An advantage of the invention is the possibility to define and carefullycontrol the low frequency pulse applied to the tissue. The real movementof the ultrasound transducer US is measured by the position sensor POS.The oscillation properties of the moving mass MOV are adjusted by thecontrol loop in order to apply the target low frequency pulse shape tothe patient's body.

In a typical transient elastography application, the low frequency pulseapplied to the patient's body has a sinusoidal shape, with a centralfrequency comprised between 1 Hz and 5000 Hz, a peak-to-peak amplitudecomprised between 10 μm and 20 mm and a duration comprised between 100μs and 20 s. The repetition rate for the ultrasound pulses is comprisedbetween 100 Hz and 100000 Hz.

According to an embodiment the peak-to-peak amplitude is comprisedbetween 50 μm and 5 mm.

The movement of the probe casing PC is transmitted to the tissue bypushing the transducer US against the tissue. The determination of thereal movement of the US transducer against tissue is difficult due tothe fact that the probe 1 is dynamically coupled also to the hand of theoperator. The movement of the hand of the operator will unavoidablymodify the shape of the low frequency pulse applied to the patient'sbody.

The present invention solves this problem by eliminating the movement ofthe ultrasound transducer US with respect to the probe casing PC and bymeasuring the position of the probe casing PC itself with a positionsensor POS. The measured position is used as a feedback for theparameters of the vibrator VIB. The parameters of the vibrator VIB arethen adjusted until the predefined low frequency pulse shape isobtained.

In other words, the probe 1 has no mechanical mobile external parts. Theprobe 1 is then an inertial probe, its movement being determined by themovement of a mass MOV placed inside the probe casing. Due to theabsence of relative movement of the ultrasound transducer US withrespect to the probe casing PC, measuring the amplitude of thedisplacement of the probe casing PC is equivalent to measuring thedisplacement of the ultrasound transducer US. The probe 1 is then ableto directly measure the shape of the low-frequency pulse applied to thetissue and to compensate an eventual motion of the operator's hand. Theabsence of external moving part eliminates also the need for frequentmechanical calibration of the probe.

According to another embodiment of the present invention, the mass ofthe moving part MOV is equal or greater than one fourth of the totalmass M of the inertial probe 1.

An advantage of this embodiment is to make it possible to effectivelycontrol the global movement of the inertial probe 1 by simply modifyingthe motion of the moving part MOV. In other words, if the mass of themoving part MOV was smaller its effect on the movement of the wholeinertial probe 1 would be smaller, due to the momentum conservation law.The control of the motion of the tip would then be less efficient.

According to an embodiment, the US transducer is a disk shape ultrasoundtransducer.

An advantage of this shape is to obtain a highly symmetric emission ofthe ultrasound shots. The high symmetric situation simplifies thecalculation of the propagation both of the ultrasound shots and of theshear wave.

According to an aspect of the invention, the force sensor FS comprises aprocessor suitable to calculate and to transmit the force applied to thesignal generator 9.

According to an aspect of the invention, when the contact ready signalis set, the ultrasound probe 1 is constructed and arranged to lights atleast one light emitting diode (LED) of said ultrasound probe 1. Infact, the lighting of a LED is used to inform the operator that theultrasound probe 1 is correctly in contact with the patient's skin sothat he stops the increase of the force applied against said patient'sskin.

According to another aspect of the invention, an acoustic indicator onthe signal generator 9 is constructed and arranged to indicate to theoperator that the contact ready signal is set.

Besides, according to an aspect of the invention, when the contact readysignal is set, the transducer US is constructed and arranged to emit theultrasound signals.

The device commands 10 are constructed and arranged to control thefrequency of ultrasound signals generated by the transducer US.

According to an embodiment the device commands 10 are also constructedand arranged to control the motion parameters of the vibrator VIBthrough a control loop using as a feedback the position measured by theposition sensor POS.

The emission and the reception of ultrasound signals by the ultrasoundtransducer US of the ultrasound probe 1 enable the acquisition of asuccession of images of a part of the medium to be analyzed. Thus, saidproduction of images is only carried out when the transducer US is incontact with a to-be-measured viscoelastic medium. Moreover, the imagesobtained by the ultrasound transducer US are in one dimension. Accordingto an aspect of the invention non-illustrated, the device DEV comprisesa plurality of ultrasound transducers US which can be positioned in anarbitrary manner, e.g., linearly (like an echographic rod) or in ahoneycomb pattern. In this manner, the device DEV permits to obtainimages in 3 dimensions. Thus, the viscoelastic properties can bemeasured in different zones of the medium to be analyzed.

Furthermore, according to an aspect of the invention, when the contactready signal is set, the display unit 7 also called “alphanumericdisplay screen” is constructed and arranged to refresh the ultrasoundimages. In fact, the alphanumeric display screen 7 of the ultrasoundprobe 1 is constructed and arranged to display the ultrasound images.The display of said images permits to assist the operator in localizingthe zone in which he wants to perform the viscoelastic propertiesmeasurements.

Otherwise, according to an aspect of the invention, when the contactready signal is set, guidance indicators are refreshed. “Guidanceindicators” mean indicators displayed to the operator to assist him inthe localization of the best measurement location.

Moreover, according to an aspect of the invention, when the contactready signal is set, the device commands 10 are constructed and arrangedto limit the access to at least one command of the device commands 10.

Besides, the signal generator 9 of the device DEV of the invention isconstructed and arranged to emit a measurement ready signal. In fact, atoo high applied force on the body can skew results of the viscoelasticproperties' measurements.

According to an aspect of the invention, the measurement ready signal isdetermined by using the force sensor FS. In fact, a measurement readysignal is generated by the signal generator 9 when the force applied bythe ultrasound probe 1 against the patient's skin is greater than aminimum measurement force threshold and smaller than a maximummeasurement force threshold.

According to an aspect of the invention, the force sensor FS comprises aprocessor suitable to calculate and to transmit the value of themeasured contact force to the signal generator 9.

According to an aspect of the invention, when the measurement readysignal is set, the ultrasound probe 1 lights at least one light emittingdiode (LED). In fact, the lighting of a LED is used to inform theoperator that he can generates a low frequency impulse.

According to another aspect of the invention, an acoustic indicator onthe signal generator 9 is constructed and arranged to indicate to theoperator the contact ready signal and/or the measurement ready signaldifferentiated by the type of sound.

According to an aspect of the invention, the vibrator VIB is put inmotion in order to apply the low frequency pulse only if the measurementready signal is issued by the signal generator 9. According to anembodiment the vibrator VIB is controlled by the device commands 10based on a feedback signal provided by the position sensor POS.

More particularly, according to an aspect of the invention, the devicecommands 10 are constructed and arranged to control the power of themechanical shear wave generated by the vibrator VIB on the patient'sskin by controlling the electrodynamic actuator VIB. Moreover, thedevice commands 10 are constructed and arranged to monitor the number ofshear waves generated in the medium.

Besides, according to an aspect of the invention, the electrodynamicactuator VIB, controlled by the device commands 10, is constructed andarranged to generate a transitory low-frequency impulse having afrequency range comprised between about 1 Hz and about 5000 Hz. The term“transitory low-frequency impulse” is understood to mean a mechanicalstress of determined duration, the frequency of which is comprisedbetween about 1 Hz and about 5000 Hz and the peak-to-peak amplitude ofwhich is comprised between about 10 μm (micrometers) and about 20millimeters, preferably between about 500 μm and about 5 mm. Theduration of this stress is comprised between about 100 μs and about 20seconds, preferably between about 5 ms and about 40 ms (milliseconds).

Thus, the electrodynamic actuator VIB, controlled by the device commands10, permits to provide a device DEV which can produce a low-frequencyvibration or stress that is perfectly controlled in time and amplitude.The form of the impulse is better controlled which enables more reliablemeasurements and thus an increase in the reproducibility of the system.Moreover, by means of the use of the controlled electrodynamic actuatorVIB, the device DEV has a reduced volume and weight.

Moreover, as specified, the ultrasound transducer US is constructed andarranged to emit and receive ultrasound signals controlled by the devicecommands 10. In particular, the device commands 10 are constructed andarranged to control the range and the frequency of the emission ofultrasound signals. Simultaneously to the generation of low-frequencyimpulse to the patient's skin, the transducer US emits and receivesultrasound signals to track the propagation of the resulting shear wave.The tracking of said shear wave permits the measurement by determiningthe viscoelastic properties of the medium. In fact, shear waves have aspecial property: their speed depends on the viscoelastic properties ofthe medium through which they have been across. The harder the liver is(and therefore the higher the level of fibrosis), the faster the shearwave propagates.

Additionally, when the device DEV comprises more than one transducer,the device commands 10 allows the control of the frequencies oftransducers.

Furthermore, according to an aspect of the invention, the ultrasoundtransducer US has an elongated shape, e.g., an oblong, rectangular,cylindrical or ellipsoid shape with a length between about 2 and about20 millimeters, preferably about 11 millimeters, and a width betweenabout 1 and about 10 millimeters, preferably about 5 millimeters.

According to an aspect of the invention, the ultrasound transducer UScan advantageously have a conical or tapered shape with an angle betweenabout 10 and about 80 degrees.

In addition, according to an aspect of the invention, the devicecommands 10 comprises a touch screen, a keyboard and optionally cursors.Moreover, the device commands 10 permits the operator to read through adisplay screen, also referred to as “operator interface”, theinformation provided by the ultrasound probe 1 linked by a flexiblecable 11 to the ultrasound probe 1.

In another embodiment of the invention, the device commands 10 permitsdeleting measurements and/or a change exam type (elastography or B-mode)and/or adding a comment or a measurement and/or changing the ultrasoundgain and/or . . . .

This invention also relates to a method MET for measuring theviscoelastic properties of a viscoelastic medium having an ultrasoundsignal after ultrasound illumination with the device DEV describedabove.

FIG. 4 represents an example of steps of the method MET for measuringthe viscoelastic properties of a viscoelastic medium.

The method MET, as represented in the FIG. 3 , comprises a positioning100 of the ultrasound transducer US in contact with a to-be-measuredviscoelastic medium.

Subsequently, the method MET comprises an application of a first force101 by an operator against the to-be measured medium by the ultrasoundprobe 1. Generally, for an assessment of liver fibrosis, the ultrasoundtransducer US applies a force on the portion that covers the ribs.

According to another aspect of the invention, the method MET comprises ameasure of the first force 102 applied by the ultrasound probe 1 againstthe to-be measured medium followed by a comparison of the measurement ofthe first force with the minimum contact force threshold. According toan aspect of the invention, the minimum force contact threshold is equalto 0.5 N.

According to another aspect of the invention the method MET comprises astep 103 of comparing the measured contact force with the maximumcontact force threshold.

Then, the method MET comprises a generation of a contact ready signal104 by the signal generator 9 based on the result of the comparison ofthe measured contact force with the predefined force thresholds. Weremind that the contact ready signal is set when the ultrasound probe 1is in contact with the medium to be analyzed. The contact ready signalis issued when the contact force is higher than a minimum contact forcethreshold. According to an embodiment the contact ready signal is issuedwhen the contact force is comprised between a minimum and a maximumcontact force threshold.

According to an aspect of the invention, the method MET comprises a stepof lighting 105 at least one light-emitting diode LED of the ultrasoundprobe 1, said LED being lighted only when the contact ready signal isset.

According to an aspect of the invention, the method MET comprises afurther step of activating the ultrasound signals 106. Thus, theultrasound transducer US emits and receives ultrasound signals toproduce images of the region of interest. We note that the ultrasoundtransducer US activates the emission and the reception of the ultrasoundsignals only when the contact ready signal is set.

According to an aspect of the invention, the method MET comprises a step107 a of displaying localization means only if the contact ready signalis issued. Localization means are tools used by an operator in order tolocate the to-be-measured viscoelastic tissue; example of localizationmeans are imaging, guidance tools or other indicators;

According to an aspect of the invention, the method MET comprises a stepof refreshing ultrasound images 107, said ultrasound images beingdisplayed by a display unit only when the contact ready signal is set.

According to an aspect of the invention, the method MET comprises,afterwards, a step of guidance 108 by guidance indicators, said guidanceindicators being refreshed only when the contact ready signal is set.

According to an aspect of the invention, the method MET comprises, astep of limitation an access to at least one command 109 offered by thedevice commands 10 when the contact ready signal is set.

According to an aspect of the invention, the method MET comprises a stepof activation a probe memory write 110 by the ultrasound probe 1 whenthe contact ready signal is set.

The method MET comprises an application of a second force 111 by theoperator against the medium by the ultrasound probe 1, the second force111 being superior to the first force 101. According to another aspectof the invention, the method MET comprises a measure of the second forceapplied by the ultrasound probe 1. Then, a comparison of the measurementof the second force value with the minimum measurement force thresholdis realized. A comparison of the second force value with the maximummeasurement force threshold can also be done.

According to an aspect of the invention, the minimum measurement force 5threshold is equal to 4.0 N. According to an aspect of the invention,the maximum measurement force threshold is equal to 8.0 N.

According to an aspect of the invention, the method MET comprises ageneration of a measurement ready signal 111 by the signal generator 9if the measured second force is greater than a minimum measurement forcethreshold.

According to an embodiment of the invention the signal generator sets111 a measurement ready signal when the measured second force iscomprised between the minimum and the maximum measurement forcethreshold.

According to an aspect of the invention, the method comprises a step 112of lighting at least one light-emitting diode LED of the ultrasoundprobe 1 when the measurement ready signal is set.

According to an aspect of the invention, the method MET comprises a step113 of generation of a low-frequency impulse to the patient's skin. Thelow-frequency impulse is generated by a movement of the whole probe 1,said movement being generated by the electrodynamical actuator VIB. Saidelectrodynamic actuator VIB is controlled by the device commands 10. Theapplication of the low frequency pulse according to the step 113 istriggered only if the measurement ready signal issued by the signalgenerator 9 is set.

According to an aspect of the invention, the method MET comprises afurther step of emission and reception 114 of ultrasound signals by theultrasound transducer US to track the propagation of the shear waveproduced by the low frequency-impulse generated 113 to the patient'sskin. In fact, shear waves propagation speed depends on the viscoelasticproperties of the propagation medium. The harder the liver is, thefaster the shear wave propagates. The displacements caused in the liverduring the propagation of the shear wave are measured by ultrasoundtransducer US using correlation techniques on the Radio Frequencyultrasound signals.

Besides, according to an aspect of the invention, the method METcomprises a step 115 of deactivation of a combined modality of thedevice DEV when the contact ready signal is set. Here, a “combinedmodality” means another type of exam combined within the device. Forinstance, the combined modality is an electrocardiography ECG or aB-mode ultrasound imaging. In a non-limiting embodiment of theinvention, the device DEV has two probes, one for each modality: a firstultrasound probe for elastography and a second ultrasound probe forB-mode imaging which is called the imaging probe. In that case, thecontact-ready signal is used to activate or deactivate one of theseprobes. It is very important for security reasons to not exceed themaximum acoustic output power that is allowed for a given application.Therefore, it is important that the ultrasound signals are not emittedsimultaneously by the two probes of the device DEV. Moreover using thetwo probes (ultrasound probe for elastography and imaging probe)simultaneously would definitively result in artefacts on both ultrasoundacquisitions as signals from probes would interfere. In a non-limitingembodiment of the invention, when the contact ready signal is set, onemodality is deactivated while the other modality is activated.

The method MET comprises a step of measuring the viscoelastic properties116 of the medium analyzed as described in the US patent No2005/0203398.

Moreover, according to an aspect of the invention, an elasticintermediary medium transparent to ultrasound (not represented in thefigures) is positioned between the device DEV and the patient's skin.According to an aspect of the invention, the intermediary medium is asynthetic polymer of the polyacrylamide type. Moreover, an adhesivematerial or a glue can be placed between the intermediary medium and themedium under study in a manner to obtain either a sliding interface or alinked interface. Besides, the intermediary medium is innovative becauseit is not only transparent for ultrasound, but also for low-frequencywaves. The intermediary medium is selected in a manner so as to presentan elasticity close to that of the medium under study in a manner toadjust the impedance and thereby enable a maximum of energy to betransmitted to the medium under study. The intermediary medium can alsobe compressed such that its module of elasticity which varies in anonlinear manner becomes close to that of the medium under study. Thislast proposition is moreover an original technique for measuring theelasticity of the medium: it comprises modifying the elasticity of theintermediary medium until a maximum of energy is transmitted. Theelasticity attained is then close to that of the medium.

According to an aspect of the invention, the ultrasound probe 1 is usedin standard echographic mode in a manner to acquire typically 20ultrasound signals per second of the tissue or the medium. The envelopeof these ultrasound signals is displayed on the alphanumeric displayscreen 7. The current signals are coded in gray level and in logarithmicscale to form an image called A-mode image. The signals can be placedside by side to constitute an image called M-mode image which containsthe ultrasound signals acquired during a given period of time, forexample 5 seconds. According to an aspect of the invention, theultrasound probe 1 is equipped with a positioning system to know thepositions at which the signals are acquired and thereby reconstitute theimage of the medium to be measured when the operator slides theultrasound probe 1 on the surface of the tissues or the medium. Besides,according to an aspect of the invention, the alphanumeric display screen7 refreshes ultrasound images only when the contact ready signal is set.According to another aspect of the invention, the ultrasound images areonly displayed at the device commands' 10 screen.

The invention claimed is:
 1. A device for measuring viscoelastic properties of a viscoelastic medium having an ultrasound signal after being subjected to ultrasound pulses, the device comprising: a probe extending along a longitudinal axis and being adapted to carry out transient elastography measurements, the probe comprising: a probe casing; at least one ultrasound transducer arranged at a tip of the probe and adapted to generate ultrasounds, and a vibrator arranged in the casing and adapted to generate a low-frequency wave, wherein: the vibrator is arranged to induce a movement of the probe casing along the longitudinal axis, the at least one ultrasound transducer is bound to the probe casing with no motion of the at least one ultrasound transducer with respect to the probe casing, a component is connected in fixed relation with the at least one ultrasonic transducer such that the low-frequency wave generated by the vibrator is transmitted via the component to the at least one ultrasonic transducer to generate a shear wave transmitted to the viscoelastic medium by the at least one ultrasonic transducer, wherein the transient elastography measurement is carried out by generating the shear wave in the viscoelastic medium by emitting the low-frequency wave using the vibrator and transmitted to the viscoelastic medium via the at least one transducer and by emitting, with the at least one ultrasound transducer, a plurality of ultrasounds pulses to track a propagation of the shear wave in the viscoelastic medium.
 2. The device of claim 1, wherein the at least one ultrasound transducer and the vibrator are aligned along the longitudinal axis of the probe.
 3. The device of claim 1, wherein the component comprises a coil of the vibrator.
 4. The device of claim 3, wherein the vibrator further comprises a movable permanent magnet.
 5. The device of claim 1, wherein the component is connected in fixed relation to a portion of the vibrator.
 6. The device of claim 1, wherein the component comprises a bar.
 7. The device of claim 6, wherein the bar is connected to the vibrator and through which the low-frequency wave is transmitted to the at least one ultrasonic transducer via the fixed relation of the bar with the at least one ultrasonic transducer, the bar being included within the casing.
 8. A method for measuring viscoelastic properties of a viscoelastic medium, the method comprising: positioning a probe in contact with the viscoelastic medium, the probe extending along a longitudinal axis and being adapted to carry out transient elastography measurements and comprising a probe casing, at least one ultrasound transducer arranged at a tip of the probe and adapted to generate ultrasounds, and a vibrator arranged in the casing and adapted to generate a low-frequency wave, and triggering a transient elastography measurement by generating a shear wave in the viscoelastic medium by emitting the low-frequency wave using the vibrator that is transmitted to the viscoelastic medium via the at least one transducer and by emitting, with the at least one ultrasound transducer, a plurality of ultrasounds pulses to track a propagation of the shear wave in the viscoelastic medium, wherein: the vibrator is arranged to induce a movement of the probe casing along the longitudinal axis, the at least one ultrasound transducer is bound to the probe casing with no motion of the at least one ultrasound transducer with respect to the probe casing, and a component is connected in fixed relation with the at least one ultrasonic transducer such that the low-frequency wave generated by the vibrator is transmitted via the component to the at least one ultrasonic transducer to generate a shear wave transmitted to the viscoelastic medium by the at least one ultrasonic transducer.
 9. The method according to claim 8, wherein the at least one ultrasound transducer and the vibrator are aligned along the longitudinal axis of the probe.
 10. The method of claim 8, wherein the component comprises a coil of the vibrator.
 11. The method of claim 10, wherein the vibrator further comprises a movable permanent magnet.
 12. The method of claim 8, wherein the component is connected in fixed relation to a portion of the vibrator.
 13. The method of claim 8, wherein the component comprises a bar.
 14. The method of claim 8, wherein the bar is connected to the vibrator and through which the low-frequency wave is transmitted to the at least one ultrasonic transducer via the fixed relation of the bar with the at least one ultrasonic transducer, the bar being included within the casing. 